Acids are materials that have certain
properties in common. Bases (also called
alkalis) are other substances with a different set of properties.
In these experiments, you will investigate some of these
properties with materials that are found around your home. In
addition, you will learn how chemists use the pH scale
to describe acids and bases.

The most striking property of both acids and bases is their
ability to change the color of certain vegetable materials. A
common vegetable whose color responds to acids and bases is red
cabbage. The first step in this experiment is to prepare an
extract of red cabbage, so you can investigate its color changes.
Place about 500 milliliters (2 cups) of red cabbage cut into
2.5-centimeter (1-inch) cubes into a blender or food processor.
Add about 250 milliliters (1 cup) of water and blend the mixture
until the cabbage has been chopped into uniformly tiny pieces.
Strain the mixture by pouring it through a sieve. This strained
liquid, the red-cabbage extract, will be used for exploring acids
and bases.

Examine the label of a bottle of white vinegar. The label
probably says that it contains acetic acid. This indicates that
vinegar is an acid and has properties of an acid. Let's see what
effect an acid has on the color of the red cabbage extract. Pour
125 milliliters (½ cup) of vinegar into a colorless drinking
glass. Add 5 milliliters (1 teaspoon) of red cabbage extract,
stir the mixture, and note its color. What is the color of the
mixture. (Write your answer in the box.)

The color of the cabbage extract with vinegar is the color the
extract has when it is mixed with an acid. Save the mixture in
this glass to use as a reference in the rest of the experiment.

Now examine the effect of laundry ammonia on the color of red
cabbage. Pour 125 milliliters (½ cup) of laundry ammonia into
another colorless drinking glass. Add 5 milliliters (1 teaspoon)
of red cabbage extract and stir the mixture. Write the color of
this mixture in the box.

Laundry ammonia is a base (alkali). The color of this mixture
is the color of cabbage extract when it is mixed with a base. The
color of cabbage extract indicates whether something mixed with
it is an acid or a base. Cabbage extract can be called an acid-base
indicator. Save the mixture in this second glass to use
as a reference.

Now test the properties of a solid, baking soda. Place 5 cubic
centimeters (1 teaspoon) of baking soda in a glass and add 125
milliliters (½ cup) of water. Stir the mixture until the baking
soda has dissolved. Then, add 5 milliliters (1 teaspoon) of red
cabbage extract to the solution. Write the color of the mixture
in the box.

The color obtained with baking soda is different from the
color obtained with vinegar and from the color obtained with
ammonia.

Red cabbage extract can indicate whether a
substance is an acid (like vinegar) or a base (like ammonia). It
can also show how strong an acid or a base a substance is.
Chemists use the pH scale to express how acidic
(like an acid) or basic (like a base) a substance is. A pH value
below 7 means that a substance is acidic, and the smaller the
number, the more acidic it is. A pH value above 7 means that a
substance is basic, and the larger the number, the more basic it
is. Red cabbage extract has different colors at different pH
values. These colors and approximate pH values are:

approximate pH:

2

4

6

8

10

12

color of extract:

red

purple

violet

blue

blue-green

green

Based on this information, what is the approximate pH of
vinegar?

What is the approximate pH of ammonia?

What is the approximate pH of the baking soda mixture?

Use the instructions for testing vinegar and ammonia to test
the pH of several other nearly colorless liquids, such as
lemon-lime soft drink (Sprite or 7-Up) and lemon juice. Record
your observations. Liquids that are white, such as milk, can be
tested in the same way. You can also test solids that dissolve in
water by following the instructions for baking soda. This will
also work with viscous liquids such as liquid detergents. Test
other substances around the house, such as sugar, table salt,
shampoo, hair rinse, milk of magnesia, antacid tablets, and
aspirin.

Material

Extract color

pH

Material

Extract color

pH

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CAUTION: Some household products can cause skin
irritations. Do not allow these to contact skin; rinse thoroughly
with water if they do.

For additional information, see CHEMICAL
DEMONSTRATIONS: A Handbook for Teachers of Chemistry,
Volume 3, by Bassam Z. Shakhashiri, The University of
Wisconsin Press, 2537 Daniels Street, Madison,
Wisconsin 53704.